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Questions and Answers
What is the primary purpose of Spanning Tree Protocol?
What is the primary purpose of Spanning Tree Protocol?
What occurs when there are redundant paths in a network?
What occurs when there are redundant paths in a network?
What is the function of the 'root bridge' in an STP network?
What is the function of the 'root bridge' in an STP network?
What tool can be used to capture and display network traffic?
What tool can be used to capture and display network traffic?
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What is the consequence of a layer 2 loop in a network?
What is the consequence of a layer 2 loop in a network?
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How is the root bridge elected in an STP network?
How is the root bridge elected in an STP network?
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What is stored in the ARP cache?
What is stored in the ARP cache?
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What command can be used to view the ARP cache?
What command can be used to view the ARP cache?
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What is the purpose of a bridge ID in STP?
What is the purpose of a bridge ID in STP?
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What is the result of a broadcast sent on a network with redundant paths?
What is the result of a broadcast sent on a network with redundant paths?
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What is the primary purpose of Spanning Tree Protocol (STP) in a network?
What is the primary purpose of Spanning Tree Protocol (STP) in a network?
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What determines the cost of reaching the root bridge in a Spanning Tree instance?
What determines the cost of reaching the root bridge in a Spanning Tree instance?
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What happens to ports that lose the election to be designated ports in a Spanning Tree instance?
What happens to ports that lose the election to be designated ports in a Spanning Tree instance?
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What is the Bridge ID composed of in a Spanning Tree instance?
What is the Bridge ID composed of in a Spanning Tree instance?
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What is the purpose of the root port on a non-root bridge in a Spanning Tree instance?
What is the purpose of the root port on a non-root bridge in a Spanning Tree instance?
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What command is used to view all Spanning Tree instances on a switch?
What command is used to view all Spanning Tree instances on a switch?
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What happens when a non-root switch receives BPDUs on multiple links with the same cost and Bridge ID?
What happens when a non-root switch receives BPDUs on multiple links with the same cost and Bridge ID?
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What is the effect of using the command 'spanning-tree vlan root primary' on a switch?
What is the effect of using the command 'spanning-tree vlan root primary' on a switch?
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What is the purpose of designated ports in a Spanning Tree instance?
What is the purpose of designated ports in a Spanning Tree instance?
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How many Spanning Tree instances are there per VLAN?
How many Spanning Tree instances are there per VLAN?
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Study Notes
CCNA Sunday - Spanning Tree Protocol
- Spanning Tree Protocol (STP) is a set of rules that layer 2 switches agree on and work with to prevent layer 2 loops in a network.
- Layer 2 loops occur when there are redundant paths in a network, causing packets to loop endlessly.
- STP is important to understand because it can save a person's bacon in a production environment if they're aware of why it behaves the way it does.
ARP Cache and Layer 2 Addresses
- ARP (Address Resolution Protocol) cache is a table that stores the layer 2 addresses associated with IP addresses on the same local network.
- The ARP cache can be viewed using the
arp -acommand. - When a device tries to reach another device on the same network, it sends an ARP request to get the layer 2 address associated with the IP address.
Protocol Analyzer and Capturing Traffic
- A protocol analyzer is a tool that captures and displays network traffic.
- Wireshark is a popular protocol analyzer that can capture traffic on a local interface.
- Capturing traffic can help identify problems in a network, such as layer 2 loops.
Layer 2 Loops and Broadcasts
- A layer 2 loop occurs when a broadcast is sent on a network with redundant paths.
- The broadcast is forwarded by each switch, causing the packet to loop endlessly.
- This can be a problem because it can cause network congestion and redundancy.
Spanning Tree Protocol (STP)
- STP is a protocol that prevents layer 2 loops in a network.
- STP works by electing a "root bridge" switch, which is the central switch that all other switches communicate with.
- Each switch has a unique bridge ID, and the switch with the lowest bridge ID becomes the root bridge.
Bridge ID and Root Bridge Election
- The bridge ID is a unique number assigned to each switch.
- The bridge ID is used to elect the root bridge in an STP network.
- The switch with the lowest bridge ID becomes the root bridge.
Determining the Root Port
- Once the root bridge is elected, each switch determines its root port.
- The root port is the port that is closest to the root bridge.
- Each switch has only one root port.
Cost of Reaching the Root Bridge
- Each switch calculates the cost of reaching the root bridge.
- The cost is determined by the number of hops to reach the root bridge.
- The switch with the lowest cost becomes the root port.### Spanning Tree Protocol (STP)
- Purpose: prevent layer 2 loops in a network
- Operation:
- Elects a root bridge based on the lowest bridge ID
- Non-root bridges calculate their root port based on the lowest cost to reach the root bridge
- Each segment has one designated port that forwards traffic away from the root bridge
- Blocking occurs on ports that lose the election to be designated ports
Root Bridge Election
- Based on the lowest bridge ID
- Bridge ID consists of:
- Priority (default: 32768)
- VLAN number
- Base MAC address
Root Port
- The port on a non-root bridge that has the best cost to reach the root bridge
- Always forwarding towards the root bridge
- Each non-root bridge has one root port
Designated Port
- The port on a segment that forwards traffic away from the root bridge
- Can be multiple designated ports on a switch
- Each segment has one designated port
- Always forwarding away from the root bridge
Blocking
- Occurs on ports that lose the election to be designated ports
- Prevents layer 2 loops in the network
- Blocking ports do not forward traffic
Spanning Tree Instances
- One instance per VLAN
- Use the command "show spanning-tree" to view all instances
- Use the command "show spanning-tree vlan " to view a specific instance
Verifying Spanning Tree
- Use the command "show spanning-tree" to verify the root bridge, root port, and designated ports
- Verify the bridge ID, cost, and port status on each switch### Spanning Tree Protocol (STP)
- Purpose: Prevents Layer 2 loops in a network
- Works by electing a root bridge and blocking redundant paths
- Uses Bridge Protocol Data Units (BPDUs) to communicate between switches
Root Bridge Election
- The switch with the lowest Bridge ID wins the election
- Bridge ID is a combination of priority, VLAN, and base MAC address
- If two switches have the same Bridge ID, the one with the lowest MAC address wins
Spanning Tree Operations
- The root bridge forwards on all its ports (designated ports)
- Non-root switches choose one port to forward on (root port) based on cost
- If costs are equal, the switch chooses based on Bridge ID
- If Bridge IDs are equal, the switch chooses based on advertised port ID
Tiebreaker Scenarios
- If a non-root switch receives BPDUs on multiple links with the same cost and Bridge ID, it chooses the link with the lowest advertised port ID
- This ensures that only one port is forwarding on each network segment
Verification and Troubleshooting
- Use
show spanning-treecommand to verify spanning tree topology - Use
show interface trunkcommand to verify VLANs allowed on trunk and being forwarded by spanning tree
Spanning Tree Configuration
- Use
spanning-tree vlan root primarycommand to make a switch the root bridge - This command lowers the priority of the switch to make it win the election
Spanning Tree Protocol (STP)
- STP is a set of rules to prevent layer 2 loops in a network.
- Layer 2 loops occur when there are redundant paths in a network, causing packets to loop endlessly.
ARP Cache and Layer 2 Addresses
- ARP cache stores layer 2 addresses associated with IP addresses on the same local network.
- The ARP cache can be viewed using the
arp -acommand.
Protocol Analyzer and Capturing Traffic
- A protocol analyzer is a tool that captures and displays network traffic.
- Wireshark is a popular protocol analyzer that can capture traffic on a local interface.
Layer 2 Loops and Broadcasts
- A layer 2 loop occurs when a broadcast is sent on a network with redundant paths.
- The broadcast is forwarded by each switch, causing the packet to loop endlessly.
STP Election
- STP elects a "root bridge" switch, which is the central switch that all other switches communicate with.
- Each switch has a unique bridge ID, and the switch with the lowest bridge ID becomes the root bridge.
Bridge ID and Root Bridge Election
- The bridge ID is a unique number assigned to each switch.
- The bridge ID is used to elect the root bridge in an STP network.
Determining the Root Port
- Once the root bridge is elected, each switch determines its root port.
- The root port is the port that is closest to the root bridge.
Cost of Reaching the Root Bridge
- Each switch calculates the cost of reaching the root bridge.
- The cost is determined by the number of hops to reach the root bridge.
STP Operation
- STP prevents layer 2 loops in a network by electing a root bridge and blocking redundant paths.
- STP uses Bridge Protocol Data Units (BPDUs) to communicate between switches.
Root Port
- The port on a non-root bridge that has the best cost to reach the root bridge.
- Always forwarding towards the root bridge.
Designated Port
- The port on a segment that forwards traffic away from the root bridge.
- Can be multiple designated ports on a switch.
- Each segment has one designated port.
Blocking
- Occurs on ports that lose the election to be designated ports.
- Prevents layer 2 loops in the network.
Spanning Tree Instances
- One instance per VLAN.
- Use the command "show spanning-tree" to view all instances.
- Use the command "show spanning-tree vlan " to view a specific instance.
Verifying Spanning Tree
- Use the command "show spanning-tree" to verify the root bridge, root port, and designated ports.
- Verify the bridge ID, cost, and port status on each switch.
Spanning Tree Configuration
- Use
spanning-tree vlan root primarycommand to make a switch the root bridge. - This command lowers the priority of the switch to make it win the election.
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Description
Test your knowledge of Spanning Tree Protocol (STP) and ARP Cache, essential concepts in networking and computer systems.
